Locks of the type to which the subject invention relates typically employ dial cams attached to a spindle and a dial or dial knob as well as lock levers or bolt levers, which are selectively engagable with the dial cam, to effect the withdrawal of the bolt to cause the lock to unlock. The bolt levers typically are moved to engage the dial cam by gravity or various mechanisms; such mechanisms include solenoids, springs and slides.
The solenoids and/or springs may be used to physically displace the lock lever or bolt lever while the slides typically are engaged with the bolt lever and cause the bolt lever to be pivoted about its connection to the bolt and into engagement with the dial cam; in some instances gravity is used to cause the displacement of the bolt lever.
The type of bolt lever mechanism where the bolt lever must be moved to engage the drive cam presents several aspects which may prove to be disadvantageous with respect to maintaining security of the lock. Bolt levers engaging dial cams for purposes of bolt withdrawal are subject to inadvertent dislocation of the lever, thereby inadvertently connecting the bolt lever with the dial cam and permitting opening of the lock without entry of a valid combination. Additionally, the contact of the bolt lever with the drive cam through the normal operation of the lock may provide characteristic sounds or noise which may be analyzed to disclose the relative position of elements within the lock, thus aiding in the defeat of such a lock. Further, the bolt lever type bolt withdrawal mechanism may be subject to end bolt pressure or end bolt force causing the bolt lever to be displaced under the influence of forces exerted on the end of the bolt, thus yielding to the end bolt pressure and allowing the lock to be physically forced opened.